Solder is an almost ubiquitous constituent of, e.g., electronic and optoelectronic systems. Among the many available solder alloys, the lead-tin (Pb-Sn) solder system is by far the most commonly used in industry today. However, even that mature solder system still has shortcomings, including joint failures. Such failures are frequently caused by creep and thermal fatigue. Stresses induced during thermal cycling because of thermal expansion mismatch are usually unevenly distributed, causing local recrystallization and microstructural coarsening. Such structural coarsening can lead to thermal fatigue crack growth and solder joint failure. Such fatigue and creep problems can be reduced by stabilizing a fine, mechanically strong microstructure and preventing structural coarsening through dispersion of fine particles (e.g., Cu.sub.6 Sn.sub.5) in the solder matrix. See, e.g. R. B. Clough, et al., Proc. NEPCON WEST '92, Feb. 23-27, 1992, Anaheim, Calif. Cahners Exposition Group, 1992, "Preparation and Properties of Reftowed Paste and Bulk Composite Solder", p. 1256. However, providing a uniform mixture of fine particles and molten solder is not an easy task as the density difference between the dispersoid particles and the molten solder (e.g., the density of Sn-Sb solder is approximately 7.2, that of Pb-Sn approximately 8.5, and that of Fe is approximately 7.9) often causes gravity-induced segregation of the particles, resulting in a macroscopically nonuniform particle distribution.
Agglomeration and coarsening of the particles is another frequently encountered problem. Because of these problems relatively large amounts (e.g. 10-40% by volume) of dispersoid typically is required in prior art compositions to obtain detectable strengthening and improvement in creep resistance. The presence of large amounts of dispersoid particles, however, tends to cause deleterious side effects such as loss of molten solder fluidity and porosity trapping. The use of other dispersoid particles in the Pb-Sn solder has been disclosed by, e.g., J. L. Marshall et al., Proc. NEPCON WEST, "Microcharacterization of Composite Solders", p. 1278, (1992), however, no significant improvement in creep resistance was reported.
A further shortcoming of Pb-Sn solder is its toxicity. Because pending legislation and EPA regulations threaten to make the use of Pb in solder much more expensive, or even ban the use of Pb-bearing solders altogether, there has been increasing interest in the development of Pb-free solders to replace currently used Pb-Sn alloys. Known Pb-free solders, however, have shortcomings in a number of material properties. These shortcomings also increase the likelihood of solder joint failures during both assembly and service by causing broken parts, misalignments, open circuits, shorts, or noisy connections.
This application discloses solder materials, including Pb-free solder materials, with improved properties, exemplarily improved mechanical strength: and creep resistance.